The present invention generally relates to an electrophotographic image forming apparatus, such as a copying machine, a facsimile machine or a laser beam printer.
There is known a laser beam printer which has a paper transport path on which paper is transported toward an upper circumferential portion of a photosensitive drum. The laser beam printer has a rotatable upper housing portion and a stationary lower housing portion. The paper transport path is located at a boundary between the upper and lower housing portions of the laser beam printer. The rotatable upper position is rotatably supported at an end portion of the stationary lower housing portion. The rotatable upper housing portion includes a suction transport unit, which sucks paper by a negative pressure and transports the same.
An image formed on the photosensitive drum is transferred to a downward surface of the paper. Thus, it is possible to eject sheets of paper in order of page to a paper eject tray. It is also possible to prevent toner particles which are not fixed on the photosensitive drum from falling to paper and being contaminated. When the rotatable upper housing portion of the laser beam printer is opened, the paper transport path is exposed. Thus, it is possible to remove jammed paper on the paper transport path. Further, since paper is transported by the suction transport unit so that it is maintained on an upper side of the paper transport path in the sucked state, it is possible to prevent paper from hanging down. If paper hangs down, a leading end of the paper is not inserted into a nip portion of a fixing roller so that paper is jammed. Further, there is also a possibility that a friction is caused between a trailing end of the paper and the photosensitive drum so that an image on the photosensitive drum which has not yet been transferred to paper is damaged.
However, the above-mentioned laser beam printer has disadvantages described below. An error in positioning the paper transport surface of the suction transport unit with respect to the paper transport path is liable to occur in the state where the rotatable upper housing portion is closed so that it is completely in contact with the stationary lower housing portion. This is caused by a play which occurs between the upper and lower housing portions of the laser beam printer by an abrasion of a fixing member which rotatably fixes the rotatable upper housing portion to the stationary lower housing portion and/or a looseness thereof. The positioning error of the paper transport surface of the suction transport unit with respect to the paper transport path prevents paper from being smoothly transported and causes a deterioration of transferred images and paper jam.